Photolithography is widely used in the fabrication of semiconductor devices. Briefly, and in general terms, an optical path, such as one including an illumination source and a patterning tool (e.g., a mask or reticle), and a projection optical system (“POS”) are used to provide patterned radiation onto a semiconductor material or material formed on semiconductor material, such as to form features of a device. The resolution and focus of the patterned radiation receive greater attention as feature size and line widths continue to decrease.